Hearthpwn

Why the spelldamage? If you are aiming for combo with nova it is either way too late or too incosistent. 

Depends on what your view on the game is. For me, card games are best when everyone sticks to fun concepts/themed decks and disregards win rating a bit. This can only be emulated with friends though. If you are playing to win, you have to go with the strong meta cards.

I would like to add that besides buffing/nerfing another factor that is important to class balance is class exclusivity. You can theoretically keep slightly overpowered cards in an area a class is lacking or slightly underpowered that combo well with a class in the game as well, as class exclusive. You can i.e make your overpowered unique charge-divine shield commander paladin only so that he doesn't flood all decks and buff rocketeer a bit as a charge option for the rest of the classes. You can make the invisible magma rager a warrior staple by keeping his stats as they are and adding the usual flame-dude effect of pyromancer and barron geddon: "deal 1 damage to ALL other minions". BAM, you got a 5 attack minion on the class that plays warsong commanders and has the charge spell that synergises with enrage and fuels battle rage. Something to consider.

And this isn't the only card that needs a buff. All people always talk about what card is OP and needs a nerf and never talk about what cards are never seen. I mean, does anyone know the card named Magma Rager? Wisp? Angry chicken? The 5-2 reckless rocketeer that costs same as Argent Commander? (who would EVER pick rocketeer over commander is beyond me). Let's start pumping out ideas for bad cards too.

In a late game buff-centric druid deck it works and it is the only deck I'm running him in. Your minions come bundled with abilities your opponent can't steal, and even if you haven't used your Marks of the wild and other buffs, if you already have the stronger board they are useless for the opponent. 

Now its weirder. You got 2 twilight drakes that require a ton of cards in your hand to gain value in a deck without a single draw card and with a ton of low cost minions you want to play early! In fact, taking out the enforcers left all your minions from 1-4 cost. Since you play no buff minions the only way you can gain damage is by inner fire or lightwarden which won't be enough and you have an excess of removal in a deck theoretically about minion buffing. Either drop the whole Divine spirit inner fire idea and go full healing adding injured blademasters circle of healings etc, or drop Lightwardens and go for buffing. Twilight Drakes won't work in this deck. 

Minion controlling needs consistency as a mechanic besides a nerf in Mind Control. Shadow madness steals the minion for one turn but lets you attack with it while other steal minion abilities don't. I know it would be overpowered, but since you don't play the minion you stole from your hand that turn it doesn't make sense to be asleep. Make it silenceable, 10 mana, steal and use. Uber late game card that still has an obvious counter that doesn't 100% negate it since you do get a turn with what you stole. Accordingly nerf mana cost for mind control minions (Sylvanas and Cabal Shadow Priest, though Sylvanas is already prone to silence so might not be necessary). 

You did VERY well to drop the priestesses. My own view on this kind of decks (Divine Shield + Inner Fire combos) is the following:

You need high hp minions to make best use of inner fire without necessarily having to use BOTH Inner Fire AND Divine Spirit (as Imho already told you). By default, the only minion you should buff with 2 cards is Lightspawn if it is left untouched, and this only with dual Spirit. All other minions is probably a no-go because killing one minion makes it a 3 for 1 trade. Lightwell is one such minion with high hp. Shieldbearer can work as well. Moghu' shan Warden is the other choice. Very underrated in such type of decks it can even go 14-14 for the win, and 7-7 taunt isn't bad either. The odd man in your list is Lightwarden. It is a fragile card and you typically want to keep board presence from turn 1 till you win. Monsters that are easily picked off are forbidden (see priestess). Overrated card, I would drop it. 2x Shattered Sun Clerics is a MUST, since you want your minions to stick on the board. Buffing and healing is the name of the deck. One Defender of Argus is never bad as well, offers attack to your lightwells and shieldbearers that need it. And since we are talking about needing attack, I do get your initial thought with priestess, but you chose the wrong buff card. Swordsmith adds damage to your high hp-low damage minions and has 3 hp to stay on the board with the addition of Power word shield or heals. Finally, this is the type of deck that revolves around strong minions. If they stick enough to snowball, you win. If they don't its already over. You have no space for Shadow Word cards in there, perhaps 1 death or 1 big-game hunter. Holy Fire is also redundant. Feel free to experiment! 

Thought of Auchenai but the reason I didn't finally go for it is that it needs cards to synergise with. Circle of Healing x 2 would be a must, if you go for circle then you probably go Injured Blademaster as well for the obvious combo and plus Holy Fire and Holy Nova backfire with her. Not to mention when you use Shadowform it has no use over your racial either. I think Auchenai is a way to play shadow-ish deck without shadowform and having to drop the heals. 

I wanted to make a Shadow Priest deck for quite some time but I lacked the cards for it. Before I start explaining my views on a shadow deck and asking for your advice I will set a couple of "rules" first:

a) I don't care to create a deck with a very good/great win rate. I'm happy with an average/decent one as long as we deviate from the usual shit we see in every deck (Argus, Clerics, Dwarves, Argent Comanders etc).

b) I want to keep it fairly close to a shadow theme. I don't mind a few odd cards but not too many. 

In one sentence, I want an acceptable shadow themed priest deck that disregards the meta because let's be honest if I wanted something related to the meta I wouldn't make a shadow priest. So, on with my views:

-Shadow Priest deck seems by default to be aiming for late game and stalling. Why? Because Shadow Priest means Shadowform and you are most probably spending 1 of your early/mid turns if not 1 and a half playing 1 or 2 shadowform. As the strongest racial, shadowform gives you an edge over the opponent the longer the game drags. And it is meant to give you an advantage on board. If you make it to the end game you should have weakened the enemy enough to win by utilizing shadow priest's late game nuke potential. Which leads us to:

-Shadow Priest nuke: Mind Blast. A low-mana hero-only nuke makes little sense for a deck that aims for late and stalling. And indeed, as you aim for late and you can't rush you have 0 reasons to use mind blast before the time you need it to end the game. Then one sees the usual Velen+blast combo and realises that for 9 mana you have a 10 nuke, and placing a 7/7 on a board that by late game you should be not losing. Sounds good. What sounds even better is that with Mind Blast being cheap and with not so low probability to have 2 in hand by late game, it does combo well with spell power too! Priests aren't usually too much into spellpower, but a single spellpower minion can make a world of difference for you. So, what high-cost minion can make your combo more consistent by acting like a second copy of Velen? Malygos (Theorycrafting about the Malygos into Velen and 2x blast should be left out of the question because I don't see it ever happening in a real match).

-Which leads us to the real issue: As you have to reach lategame where your superior racial and end-game nuking will win you the game, your sole worry should be how to get there. And the problems start: Priest isn't exactly the fastest class there is. Priest early game minions are a problem in their own right because most of them like Northshire Cleric, Lightspawn and Lightwell are based on healing.When you switch to shadowform, on top of slowing yourself down, you also drop any healing synergies your minions have. A second problem is card draw. Thoughtsteal seems weird to play early to mid when you'll be struggling for presence and normally you will want to try and play Shadowform too, which will make it nigh impossible to keep anything on board. So what if you decide to play NOTHING early and just stall? To do that you need to keep enemy's side empty.Then we get to:

-Lack of early AoE. Holy Novas aren't enough and are a bit late imho, and while Shadow madness can in cases be effective, it does have a lot of randomness attached. But these are your only options and you have to live with them.

Considering the above, I ended up with the deck below: I HAVEN'T tested yet and I KNOW it is expensive as fuck. Plus I guarantee it won't have magnificent results. However, if anyone can be bothered to help me with expressing their views on a mostly shadow themed deck and what I can alter please feel free to.

Reasoning on the cards I picked:

2x Shadowform: No need to explain.

2x Mind Blast: The way to end games.

Velen / Malygos: The end game options that augument your Mind Blasts to 2 mana pyroblasts. 

Holy Nova / Shadow Madness: Can't do without some AoE. These are the options.

2x SW: Death / 1x SW: Pain: Seems weird that I run 2 deaths and one pain but between your racial, dual smites, shadow madness, Cabal Shadow Priest and Holy Nova theoretically smaller enemies shouldn't stay on the board for too long and pain should be saved only for stuff you either can't easily remove by damage (divine shields, shieldmasta) or to bypass shit like Acolyte's card draw. For the larger minions, there is death.

Mind Control: Just too good to pass and fits the theme.

Holy Fire: One copy as it can either remove something nasty or hit enemy hero for some direct damage. Synergises with Velen and spell damage and also heals a bit to keep you alive to reach later game. I would really love to run 2 but it's too expensive to easily combo, mind blast should be enough to finish games and PW: death is cheaper as a removal for beefy enemies.

2x Holy Smite: Can be used in conjunction with your racial to kill stuff not normally killable by priests due to the usual 4 attack window, with the bonus that if you ever pull one of these late game it can combo well with both Velen and Malygos. Until then, it can be dropped with azure drake as well for good damage.

2x Thoughtsteal: Card draw is good and there aren't limitless options.

1x Ooze: Weapon removal and a creature with enough punch to trade with something early before your Shadowform is available.

2x Faerie Dragon: You want to trade early to keep enemy board empty until your shadowform comes. Faerie Dragon can stay on the field to trade avoiding most early removal except weapons and a couple odd stuff like Arcane missiles.

2x Harvest Golem: Another cheap minion that can probably stay on the board to trade with something and will fill your side as you play Shadowform.

Thalnos / 2x Azure Drake: The best spell damage cards that also get you draws. 

2x Shieldmasta: Since we lack AoE, we need something to help stall and deal with numbers of smaller stuff. This is the best option at that, and draws enemy removal. 

2x Gnomish Inventor: Gnomish Engineer can make the early game with this deck way too hectic (as if it isn't already) because you can't afford to slow yourself down anymore early. This is Engineer a few turns late, with more meaningful stats for us.